Typically, a shoulder strap in a restraint system must be relatively long in order to extend from a person's waist to some attachment point above or behind the person. In addition there must be sufficient strap to accommodate the largest person to use the system, plus provide for some freedom of movement. Because of the length of the strap, it is normally necessary to provide a relatively strong rewinding spring which will produce sufficient torque to urge the strap into a fully retracted position.
The user is permitted to move against the retracting force, and the strap will usually be locked in response to the sensing of a danger, such as sudden extension of the strap, deceleration of the vehicle or other triggering means. It is desirable that the strap be continuously under a retracting force so that the user will be immediately restrained when the strap is blocked from further strap extension. If the strap is constantly under a relatively strong retracting force, the strap can be annoying or cause irritation to the wearer's shoulder over a prolonged period of time.
A variety of solutions have been developed but all have various disadvantages. To overcome this problem, some retractors currently on the market provide a dead zone without any retracting tension for a given amount of movement of the wearer's upper torso. A shortcoming of this approach is that the wearer's upper torso can move a considerable distance before the locking mechanism on the retractor is actuated, so that the strap is subjected to a sudden large impact due to the momentum of the extra traveling distance. This is undesirable with respect to the strap, and the wearer may be injured by the impact.
There are also systems requiring a manual adjustment to eliminate the tension, such as pushing a button or moving a clip on the strap to receive the strap tension. This of course is inconvenient and can create an unsafe situation in that many people do not realize the importance of limiting the amount of freedom with no load. Another disadvantage is that these manual approaches require readjustment for users of different sizes. Also, some wearers feel more secure with tension, and think their restraining system is not functioning if there is no tension on the strap.
Another system provides a relatively complicated electric drive to insure proper strap rewinding. A need still exists for a system which will provide greater comfort to the wearer in a simple and automatic manner while still meeting the necessary safety requirements, which are basic to the entire subject.